Eastwood sells a cleaning/sealing system that is supposed to work pretty good, but it is not cheap.
I did one 30+ years ago by dumping in about 3 pounds of 1/4-3/8 nuts and bolts and swishing them around with acetone or something like it. That one wasn't so bad and it worked well.
The tank in my current car was replaced with a new one due to the amount of rust inside, not just residue.